It is conventional practice to measure total pressure by utilizing a standard Pitot tube that is located within the flow stream of the pressure being measured. Because of their complexity, accuracy and reliability, Pitot tubes have not been widely utilized for on-board aircraft gas turbine engine sensors. While total pressure sensors are occasionally utilized for applications where there is an absence of total pressure distortions, these obviously are not used where pressure distortions are present. In a gas turbine engine inlet, where this invention is viable, the total pressure distortions preclude the use of total pressure sensors.
Another method of obtaining tool pressure is by inserting a probe in the flow stream and measuring the static pressure of that stream. This value can then be converted to the total pressure or to a close proximity thereto. Such a system is exemplified in U.S. Pat. No. 4,414,807 entitled "A Method and Apparatus for Controlling a Gas Turbine Engine" granted to W. B. Kerr on Nov. 15, 1982 and assigned to United Technologies Corporation, the assignee of this patent application. In this patented system the probe extends from the center of the engine's nose cone and extends axially forward approximately 1/2 engine diameter along the engine's axis. In this position, it is in the center of the air stream and a significant distance away from the engine's face. The measured static pressure is utilized as an input to certain controls for the engine and is converted to a usable parameter such as total pressure or engine pressure ratio and the like.
Obviously, a degree of complexity and hazard accompanies a sensor that includes a protruding probe. The probe, for example, disclosed in U.S. Pat. No. 4,414,870, supra, projects away from the front of the engine and precaution has to be taken to avoid breakage when the engine is being maintained or transported. Additionally, the probe is relatively expensive and adds to the overall weight of the engine.
I have found that I can obviate the problems enumerated above by providing a discrete number of static ports formed in the engine's case at a judicious location near the front of the engine. Locating the static ports at this particular location is contrary to heretofore practice and beliefs. It has heretofore been understood and assumed that the static pressure had to be measured some distance away from the front of the face of the engine so as to avoid the influence of engine suction and final air acceleration effects. However, in spite of these prior understandings, I have found that an accurate, reliable static pressure signal can be obtained by locating, and pneumatically averaging, a given number of static ports around the circumference of the face of the engine, as in the engine's casing, and located as proximate to the engine's face as possible. A system employing my invention has been tested under severe total pressure distortion conditions and has evidenced static pressure values well within the range of acceptability for use in a control system.